This invention relates to an electroluminescent (EL) lamp and, in particular, to a low cost inverter having a minimal number of components for converting direct current into alternating current for operating an EL lamp.
An EL lamp is essentially a capacitor having a dielectric layer including a phosphor powder which glows in the presence of a strong electric field and a very low current. The dielectric layer is held between two electrodes, one of which is transparent. Because the EL lamp is a capacitor, an alternating current (AC) must be applied to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage and the current through the EL lamp ceases.
For personal electronic devices such as wristwatches, pocket pagers, and cellular telephones, an EL lamp is driven by an inverter which converts direct current from a small battery into alternating current. In order for an EL lamp to glow sufficiently, a peak to peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder.
While there are many ways to increase voltage, e.g. by using a transformer or a voltage doubler, most applications for an EL lamp use what is known as a "flyback" or boost inverter in which the energy stored in an inductor is supplied to the EL lamp as a small pulse of current at high voltage. The inverter typically operates at high frequency (4 khz. or more) to minimize the size of the magnetics, i.e. the inductor or transformer, in the inverter.
The frequency of the alternating current through an EL lamp affects the life of the EL lamp, with frequencies below 1000 hz. being preferred. Too low of a frequency causes a noticeable flicker and low brightness. Thus, a frequency of 100-1000 hz. is preferred although a broader range is useful. Many types of inverters are commercially available for producing AC at a low frequency, i.e. a frequency of approximately 100 to 1000 hz. Frequencies above 1000 hz. can be used if the high frequency signal is modulated with a signal having a frequency of 1000 hz. or less.
It is desired to reduce the size of components as much as possible to reduce the size of personal electronic devices and to provide flexibility in the arrangement of components even if space is not critical. Reducing the number of components further reduces the size and cost of such devices and improves reliability. The number of components often can be reduced by using custom components, which cost more than standard components and may not be as conveniently obtainable as standard components.
Custom components may also enable one to improve the efficiency of an inverter, i.e. the amount of light produced by an EL lamp per unit of electrical power consumed by an inverter and lamp combination. Personal electronic devices, particularly wristwatches, use small batteries. An efficient inverter permits a brighter display, a longer battery life, or both. A brighter display also provides more flexibility in designing the display. For example, one can use black overprints on a bright lamp but not on a dim lamp. A black overprint is a graphic which covers a substantial portion of a lamp to provide a "white on black" display. A more efficient inverter can also produce a higher voltage for the EL lamp, enabling one to use EL lamps having split electrodes, i.e. the electrical equivalent of two (or more) EL lamps connected in series across a source of voltage.
To date, so far as is known, there are no commercially available wristwatches with an EL lamp powered by a single battery having a voltage of 1.5 volts or less, although there are patents describing such watches. One reason for the lack of single battery EL wristwatches is that the step-up ratio necessary for driving an EL lamp from such a low voltage is in excess of 90:1. Producing 140 volts from 1.5 volts is difficult without resorting to high current, i.e. a current greater than 50 milliamperes. Watch batteries, even if capable of providing such current, can not do so for long. The result is that a pair of batteries is used to provide at least 3.0 volts for wristwatch and other applications.
Another reason there are no single battery EL wristwatches is that many semiconductor devices behave differently when the supply voltage drops below 1.5 volts. Integrated circuits work much more slowly, if at all. Voltage drops in the inverter circuit become critical, e.g. a forward biased PN (diode) junction has a voltage drop of about 0.6 volts. Three junctions in series can not be forward biased and an inverter incorporating these junctions will not work at 1.5 volts, although the inverter may work well with a 3.0 volt supply.
FIG. 1 is a schematic diagram of a flyback inverter known in the prior art; e.g. see U.S. Pat. No. 4,527,096 (Kindlmann). When transistor 14 turns on, current flows through inductor 15, storing energy in the magnetic field generated by the inductor. When transistor 14 shuts off, the magnetic field collapses at a rate determined by the turn-off characteristics of transistor 14. The voltage across inductor 15 is proportional to the rate at which the field collapses. Thus, a low voltage and large current is converted into a high voltage at a small current.
The current pulses are coupled through diode 16 to the DC diagonal of a switching bridge having EL lamp 12 connected across the AC diagonal. Assuming that transistors 18 and 19 are conducting, the same amount of energy is supplied to lamp 12 each time transistor 14 turns off and, therefore, the voltage on the lamp is pumped up by a series of current pulses from inductor 15 as transistor 14 repeatedly turns on and off. Diode 16 prevents lamp 12 from discharging through transistor 14. If transistor 14 were switched on and off continuously, the pulses would charge lamp 12 to the maximum voltage available from inductor 15, e.g. about 140 volts. Since an EL lamp needs an alternating current or a variable direct current, the lamp would glow initially and then extinguish when the capacitance of the lamp became fully charged.
To avoid this problem, the transistors in opposite sides of the bridge alternately conduct to reverse the connections to lamp 12. The bridge transistors switch at a lower frequency than transistor 14. The four bridge transistors are high voltage components, adding considerably to the size and cost of the circuit. In addition, the circuit is not single ended, i.e. one cannot ground one side of lamp 12.
It is preferred that an inverter have a single ended output, i.e. a high voltage terminal and a ground terminal for connection to a lamp. Many inverters of the prior art require that neither electrode of an EL lamp be grounded. This often complicates making electrical connections to a lamp or making connections to several lamps.
FIG. 2 is the schematic of an inverter disclosed as prior art in U.S. Pat. No. 4,529,322 (Ueda). In this inverter, transistor 14 is switched on and off at about 8 khz. When transistor 21 is conducting, lamp 12 is discharged and the current through the lamp is variable DC, not AC. Since the EL lamp and the discharge transistor are series connected across the power supply, there will always be a residual DC bias across lamp 12 equal to the voltage at terminal 13. DC bias on an EL lamp can cause corrosion and shorting of the electrodes of the lamp, particularly at elevated temperature and humidity, decreasing the life of the lamp. In watch applications, a DC bias can be tolerated because the lamp is not used often and the life of the lamp far exceeds the life of the watch. In other applications, DC bias is a problem.
A second problem with the circuit disclosed in the Ueda patent is that transistor 21 draws current from terminal 13 through diode 16. This current is wasted since it does not contribute to powering lamp 12 and it reduces the efficiency of the inverter. The waste current is limited by resistor 22, connected between the emitter of transistor 21 and ground, but the circuit is inefficient nevertheless because the discharge of lamp 12 pushes current in the wrong (forward) direction through the power supply connected to terminal 13 and ground.
FIG. 3 is a portion of a schematic disclosed in PCT application no. PCT/US93/04698 (McLaughlin et al.) which discloses an EL lamp powered by a flyback inverter. A discharge path includes transistor 37 connected in parallel with lamp 36 and the transistor is driven at the same frequency as transistor 32. Since the charging and discharging transistors turn on and off together, one does not have the waste current as in the circuit shown in FIG. 2. However, the circuit of FIG. 3 does not use a series of pulses to pump up the voltage across an EL lamp, a single pulse is used instead. A relationship used in the art to estimate component values is EQU f.sub.h Li.sup.2 .gtoreq.f.sub.l CV.sup.2.
(f.sub.h is high frequency, L is inductance, i is current, f.sub.l is low frequency, C is capacitance, and V is voltage.) In order to produce a high voltage across an EL lamp from a single pulse, the inverter described in the PCT application must use a relatively large current, greater than 50 ma., to create a large magnetic field and the inductor must have a fairly large inductance. Since high current, high voltage components are physically large and are expensive, the inverter is not commercially practical.
FIG. 4 is a portion of a schematic of an inverter disclosed in European patent application EP 0 535 885 A2 (Komoda). Transistor 42 is driven at 250 khz. and transistor 47 is driven at 200 hz. A voltage tripler including three diodes connects EL lamp 46 to the junction of transistor 42 and inductor 41. The supply voltage is not disclosed. An output voltage of 300 volts peak to peak is alleged.
Thus, while many circuits exist in the prior art for inverting low voltage DC to high voltage AC, these circuits typically include many components, including custom semiconductor devices and custom magnetics, increasing the cost and power dissipation of the circuits. For wristwatches, pagers, and other applications where a small battery is used as the power source, it is desired to minimize power consumption and to improve the efficiency of an inverter for an EL lamp.
In view of the foregoing, it is therefore an object of the invention to provide a low cost inverter for an EL lamp, the inverter having a minimum number of components and using only commodity components, i.e. easily obtainable and inexpensive components.
Another object of the invention is to provide a small, efficient inverter for EL lamps.
A further object of the invention is to provide a low cost inverter suitable for watch, pager, and other applications wherein the power supply is a small battery having a voltage of three volts or less.
Another object of the invention is to provide a small, efficient, single ended inverter for EL lamps.
A further object of the invention is to eliminate waste current in an inverter.
Another object of the invention is to provide an inverter having few components in which efficiency is improved by coupling unused energy back to the power supply.